1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus which performs helical scanning.
2. Description of the Related Art
Helical scanning is used to acquire projection data by scanning an subject with cone beam X-rays while moving a top on which the subject is placed. The image data based on the projection data acquired by helical scanning is reconstructed in the following general sequence:    1. extracting projection data (a projection data set) necessary for reconstruction of one image;    2. performing data redundancy correction (weighting);    3. performing filtering; and    4. performing backprojection    It suffices to interchange steps 2 and 3. Consider weighting in step 2. Reconstruction processing (half scan method) uses projection data (a projection data set) in the range from (−π/2−α/2) to (+π/2+α/2), centered on a target slice position. A weighting function is designed to assign smaller weights to projection data farther from the slice position and assign larger weights to projection data closer to the slice position. When the speed of the top (helical pitch) is constant, the slice position on this weighting function matches the view center (half of the total number of views of the projection data set) of the projection data set extracted in step 1. Such a weighting function having a weight distribution symmetrical to the view center is called a shift-invariant weighting function.
As an application of helical scanning, there is available a variable speed helical scanning scheme of performing scanning while changing the speed of the top (helical pitch). FIG. 11 is a graph showing the relationship between a weighting function in variable speed helical scanning (acceleration) and the position of an X-ray tube. In a case of variable speed helical scanning (acceleration), the number of views from (−π/2−α/2) to 0 is larger than that from 0 to (+π/2+α/2). As shown in FIG. 11, therefore, when the speed of the top is changed during scanning, the slice position IS does not match a view center tc of a projection data set used for reconstruction. If the shift-invariant weighting function is applied to the variable speed helical scanning scheme in which the slice position IS does not match the view center tc, artifacts occur.